136 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



(1959), and Fjerdingstad (1961a). Fjerdingstad found that the 

 neatly-spaced microvilli of the collar in Spongilla lacustris were 

 connected by very fine lateral filaments to form a rather cohesive 

 unit. Such filaments apparently are lacking in Ephydatia fluviatilis, 

 examined by Rasmont, where the circlet of individual villi 

 nevertheless maintains a rather impressive orderliness. The 

 flagellum in both species is conventional, with an open, cylindrical 

 kinetosome like those of protozoa. According to Rasmont, no 

 root fibers are present. Additional studies of both the choanocyte 

 and the choanoflagellate are needed to permit comparison of 

 cytoplasmic structure and inclusions. 



Grasse and Denandre (1952) include in the Order Bicoecida a 

 few genera of little-known organisms showing some resemblance 

 both to the chrysomonads and to the choanoflagellates. They 

 have two flagella, one of which is always directed posteriorly 

 and attaches the cell to the bottom of its envelope, which may 

 contain iron salts. This envelope has been examined (Robinow, 

 1956) in two species oiBicoeca (Bikosoeca), in which it resembles to 

 a very striking degree the envelope of the chrysomonad Stokesiella 

 (p. 112). Like the latter it is formed of a spirally wound 

 membranous band. This is not surprising in view of the fact that 

 species of the two genera were originally placed in the same genus. 

 Unfortunately the flagella in Robinow's specimens were coiled 

 within the envelope, so that no information is available on the 

 presence or absence of mastigonemes. 



If the choanoflagellates and bicoecids can be segregated as 

 possible chrysomonad derivatives, the remaining three orders of 

 the Superorder Protomonadica, while not revealing phytoflagellate 

 affinities, do show relationship with each other and seem to 

 constitute a natural grouping. The Bodonida include the only 

 free-living species; the Trypanosomatida and the Proteromona- 

 dida are all symbiotic. Grasse (1952, pp. 47-49, 575) considers 

 that the bodonids may represent an important stem group, 

 leading through the proteromonads to the Metamonadica, and 

 possibly implicated in the ancestry of the Sporozoa and even of 

 the metazoa. As yet we have no information on the ultrastructure 

 of the Proteromonadida, but various trypanosomes and two 

 representative bodonids have been examined by electron 

 microscopists. 



